1. Field of the Invention
The present invention relates to a semiconductor integrated circuit.
This application is a counterpart application of Japanese application Serial Number 285898/1999, filed Oct. 6, 1999, the subject matter of which is incorporated herein by reference.
2. Description of Related Art
In the development of semiconductor integrated circuits, part of a circuit is sometimes common to different types of integrated circuits. Development costs can be reduced and development time shortened by having parts of the circuits in common.
In the case where part of an integrated circuit is made common, unused pads are sometimes formed within the integrated circuits.
For example, we will consider the case of two types of integrated circuits A and B, where three types of control signals S1, S2, and S3 must be input from outside to the integrated circuit A, but only control signals S1 and S2 need be input from outside to the integrated circuit B and the control signal S3 is not used thereby.
In such a case, when these integrated circuits A and B have common portions, pads P1, P2, and P3 for inputting control signals S1, S2, and S3 are formed in each integrated circuit A and B. Pads P1, P2, and P3 are all used in integrated circuit A, but only pads P1 and P2 are used in integrated circuit B and pad P3 is not used thereby.
When unused input pads are present in a conventional semiconductor integrated circuit, these unused input pads must be set to a prescribed potential. In a conventional semiconductor integrated circuit, wires or the like are therefore bonded to unused input pads as well as to used pads and potentials such as VCC and VSS are supplied thereto. The potential must be fixed because, when the potential of unused input pads is not fixed, the varying potential of the input pads due to minority carriers or the like may be handled within the integrated circuit as an input signal and cause erroneous operation of the integrated circuit.
Moreover, it is desirable that the number of pads to be bonded be reduced in view of reducing manufacturing costs for semiconductor devices and improving yields.
From this perspective, a technique for eliminating the need for the bonding of unused input pads is desirable.
It is an object of the present invention to provide a semiconductor integrated circuit that does not require the potential of unused input pads to be held at an externally supplied potential.
In order to achieve this object, the semiconductor integrated circuit relating to the present invention is provided the unique constitution described below. Specifically, the semiconductor integrated circuit relating to the present invention is provided an input circuit between an input pad and first stage input gate. In the present invention, this input circuit comprises a determining portion to determine the presence of bonding at an input pad according to the potential of that input pad, and a switching portion to output a first level potential to the first stage input gate when the determining portion determines that xe2x80x9cthere is no bondingxe2x80x9d and to connect the input pad to the first stage input gate when the determining portion determines that xe2x80x9cthere is bondingxe2x80x9d.
In a semiconductor integrated circuit relating to the present invention, the determining portion checks for bonding and the switching portion sets the potential level of the first stage input gate in the case there is no bonding. Consequently, it is not necessary to perform bonding because it is not necessary to fix the potential of the unused input pads with an externally supplied potential.